Segmentation fault when trying to validate query with unterminated recursive subquery

### Steps to reproduce:

Attempt to validate query with unterminated recursive query definition, such as
```
DEFINE SUBQUERY $foo($bar) AS
SELECT * from $bar($bar)
END DEFINE;

SELECT * from $foo($foo)
```
### Expected result

Failed validation attempt with appropriate error message

### Current result:

YQL agent terminates due to segmentation fault